It is known that liver growth is governed by two governing systems, of which one controls the growth of the volume of the liver cells and the other controls the increase in the number of liver cells (rate of proliferation). It is also known that when a portion of the liver is surgically removed (partial hepatectomy), a rate of proliferation of the liver cells much higher than normal, that is, a much greater level of cell division activity, occurs. This results in a great increase in the number of liver cells, which had been reduced by the operation, until the original number of liver cells has again been approximately attained. Such an increase in number of liver cells can also be observed in mammals in the fetal state or in the first weeks after birth. This increase takes place up to a certain number of cells. From that point, liver growth resumes by means of cell volume enlargement.
It has already been suspected that the control of the liver cell proliferation rate takes place by means of a liver cell proliferation factor or composition. However, equal consideration has been given to the possibility that there is a factor which under normal conditions inhibits cell division and, in the case of partial hepatectomy, disappears as the result of influences which are so far unknown. (See, as an example, the summary of knowledge in the field to that date in: Bucher, N. L. R. and Malt, R. A., Regeneration of Liver and Kidney, Boston: Little, Brown and Co., 1971, pp. 245 ff.)